Offsets may have different causes in a signal processing chain of an integrated circuit. Fluctuations in the supply voltage, temperature drift effects, fluctuations in process parameters during fabrication and matching problems between electronic components in the case of differential signal routing thus contribute, for example, to the occurrence of offsets.
Such direct current (DC) offsets may have undesirable effects in transmission arrangements, for example. DC offsets which occur in the baseband signal processing system of a transmitter thus result in a sinusoidal signal at the respective transmission frequency at the output of the modulator. This significantly increases error vectors in digital modulation methods. It is therefore desirable to be able to compensate for such DC offsets.
It is possible to reduce the DC offsets by introducing a compensation signal, which compensates for the offset, at the input of the baseband signal processing system of a transmission arrangement. This may be carried out, for example, when fabricating mobile radios. In this case, the compensation voltage (which is respectively needed for the purpose of compensation) for each individual device may be stored in a nonvolatile memory of the baseband chip. In the reception path of a mobile radio, a DC offset may even result in system failure since the extremely high gain in the analog baseband may result in transient spikes which may drive individual stages of the signal processing chain into saturation and may result in analog/digital converters being overdriven.
It goes without saying that DC offsets may be avoided using AC coupling, that is to say a high-pass filter having a low cut-off frequency. However, such AC coupling requires a relatively large chip area on account of the series capacitances which are normally provided for implementation. In addition, information components of useful information may be removed or filtered out in an undesirable manner.
Code division multiple access (CDMA) methods which operate in a continuous-time mode are used in modern mobile radio methods, for example the so-called Universal Mobile Telecommunications System (UMTS). Accordingly, it is not possible, in contrast to earlier mobile radio methods, for example GSM (Global System for Mobile Communication), which operate using time division multiple access, to periodically interrupt the signal chain in order to be able to carry out calibrations during transmission operation.
DC feedback affords another possible way of compensating for DC offsets. However, this principle has the disadvantage that a relatively low cut-off frequency of the high-pass filter is required in order to avoid the useful signal being excessively distorted. However, this means that the transient response time becomes relatively long, which, in turn, has adverse effects on the received signal. Another disadvantage results from transients which may occur when there is a sudden change in the gain, even in the case of high-pass filters which settle more rapidly, and may exceed the useful signal many times over. The analog/digital converter thus cannot be controlled in an ideal manner. In particular, the switching transients which have been mentioned and occur when the gain value is changed in the reception path of a transmission/reception unit result in considerably impaired electrical properties of the receiver.